Linda Carroll, a 59-year-old marriage and family therapist, told her hometown newspaper, the Gazette-Times, the widely reported rumor was "crazy" and "not true."
Carroll was thrust into the public eye last week when a London tabloid said Brando was Love's grandfather by virtue of a brief liaison with Carroll's mother, novelist Paula Fox.
The story was reprinted and broadcast worldwide.
The source of the information was said to be Carroll's upcoming memoir about her childhood. But Carroll told the newspaper her book says nothing of the sort, and Brando is definitely not her father.
Carroll said she realizes it's because of her famous daughter that the rumor has taken on a life of its own.
"It's really a juicy story about Courtney Love," she said.